The proper and efficient operation of a sewage treatment plant is dependent upon the rate at which activated return sludge is cycled in the system. The return sludge carries the necessary operating bacteria back into the system to control the rate of treatment. The best operation is obtained when one knows continuously the condition of the return sludge which is determined by measuring the amount of solids suspended in the fluid and the settling rate of the activated sludge. In the prior art the only known way to determine these factors involves a manual test in which an operator must take a sample of the sludge and the mixed liquor and run them through a prolonged settling test in which the amount of solids and the settling rate are observed by eye and recorded by hand. This operation is slow and expensive since it requires the labor of the operator. Frequently the tests simply are not performed often enough to maintain a good operating picture of the system and consequently the sewage plant never quite operates at its optimum level but is always in the process of being corrected back from extreme conditions of too much or too little activated sludge. The present invention corrects these difficulties.